The legislative branch of the United States consists of two houses: the Senate and the House of Representatives. The number of legislators that a state can send to the House of Representatives depends on the number of citizens who live in that state. To elect each of these representatives, the government of a state divides itself geographically into congressional districts. The citizens who live in each of these districts elect the politician who will serve as their congressman or congresswoman in the House of Representatives.
Congressmen and congresswomen from a particular congressional district are charged with representing the specific interests of the voters from that district. This includes voting on legislation according to the prevailing sentiment in the congressional district that elected him or her, and it often takes the form of securing federal spending that specifically benefits the district. A congressional district holds an election for its representative every other year.
Each state divides its voters geographically into congressional districts so that it has one congressional district for every seat that it has in the House of Representatives. This process is also known as “drawing the map” of congressional districts. The number of congressional districts that a state will have is determined every 10 years when the U.S. takes a census of its population. The country uses the census' count of the population in each state to determine whether each state will have more seats, fewer seats or the same number of seats in the House of Representatives.
States have the power to redraw the map of their congressional districts at any time, but most do so after the national census. Some states have their legislature draw the new maps, and other states have independent commissions that draw these maps. When a state redraws its congressional districts, it is supposed to ensure that different congressional districts contain roughly the same number of voters. It is for this reason that areas that have a low population density will have geographically large congressional districts and areas that have a high population density will have geographically small congressional districts.
Each congressional district is also supposed to contain voters who live in the same communities and have similar interests, and the district boundaries should not purposefully divide an influential group of voters. When states redraw lines to distribute such a voting group into multiple districts in which it will be reliably outvoted, it diminishes the group's political influence. This process is referred to as gerrymandering, and it is especially controversial when it reduces the political influence of minority groups.